Fuel bundle designs for boiling water nuclear reactors are known. Such fuel designs are fabricated in a standard fashion including a lower tie plate for supporting an upstanding matrix of fuel rods in side-by-side relation and permitting the inflow of water coolant into the fuel bundle. Most of the fuel rods of such a fuel bundle extend from the supporting lower tie plate to an upper tie plate. This upper tie plate serves to maintain the fuel rods in upstanding side-by-side relation and to permit the exit of water and generated steam from the fuel bundle.
The fuel bundle is typically surrounded by a fuel bundle channel, which channel surrounds the lower tie plate, extends upwardly around the fuel rod matrix, and surrounds the upper tie plate. This fuel bundle channel isolates the flow path through the fuel bundle so that water and steam generated in the interior of the fuel bundle are separate from the so-called core bypass region surrounding the fuel bundle. This core bypass region contains water moderator and occupies generally cruciform shaped volumes between the fuel rods into which control rods can penetrate for the absorption of thermal neutrons for the control of the nuclear reaction.
In operation of the boiling water nuclear reactor fuel bundles, liquid moderator--water--is introduced at the bottom of the fuel bundle through the lower tie plate. The water passes upwardly interior of the fuel bundle and performs two major functions. First, it moderates so-called fast or energetic neutrons produced in the nuclear reaction to slow or thermal neutrons need to continue the nuclear reaction. Secondly, the water moderator generates steam which is utilized for the generation of power.
It will be understood that the fuel rods interior of the fuel bundles are long slender sealed tubes containing fissionable material and are flexible. If such fuel rods were to be unrestrained, they would vibrate and even come into abrading contact with one another during the generation of steam. To restrain this tendency as well as maintain the fuel rods in their designed side-by-side spacing for efficient nuclear operation, so-called spacers are utilized. These spacers are placed at selected vertical intervals within the fuel bundle. Usually, seven evenly distributed fuel rod spacers are utilized in a fuel bundle having an overall length in the order of 160 inches. These spacers surround each individual fuel rod maintaining the precise designed spacing of the fuel rods along the entire length of the fuel bundle.
It is standard practice to improve the performance of boiling water fuel assemblies by introducing special water regions which distribute controlled amounts of water liquid moderator within the fuel assembly lattice of fuel rods. This is often accomplished by the use of hollow rods ("water rods") or other generally vertically aligned parallel flow conduits, through which substantially single phase water flows. Typically, a small amount of water is bypassed from the lower tie plate region through these water regions--more often referred to as water rods--and finally discharged out the top of the fuel bundle. These controlled water regions are particularly effective in the upper portions of the fuel assembly where neutron moderation is normally reduced by the steam which displaces the liquid water for a large fraction of the coolant flow area. However, since the introduction of such water regions occupies space that would otherwise contain more uranium fuel, the net performance benefits of the water regions are a trade-off between the positive effects of improved neutron moderation and the negative effects of decreased uranium fuel content. As a consequence, careful studies are required to establish optimum shapes and numbers of these water regions in any particular boiling water reactor fuel bundle design.
It is further standard practice for these water regions to extend upward from the bottom of the fuel assembly at the lower tie plate. This is done so that flow holes can be allowed for entry of subcooled water to the water regions from the lower tie plate at the bottom of the fuel assembly. This direct entry of subcooled water to the water regions is used in order to avoid the entry of steam into the water regions and to avoid unknown neutron moderation conditions that would result if both water and vapor were present within the water regions. Having a direct flow of subcooled water to the water regions avoids or minimizes subsequent steam formation from neutronic heating at higher elevations in the fuel bundle within the water regions.
However, extending a water region from the bottom of a fuel assembly also has a detrimental effect on the fuel assembly performance. The adverse effect results from the removal of uranium fuel in the lower region where the adverse effect resulting from the removal of fuel rods is not compensated by large benefits from increased neutron moderation. Thus, this adverse performance in the lower portion of the fuel assembly limits the overall effectiveness achieved through addition of water regions to the boiling water fuel assemblies.
So-called part length rods have been introduced into this standard fuel bundle construction. These part length fuel rods extend from the lower tie plate only partially the distance to the upper tie plate. The fuel rods typically terminate underlying the upper tie plate so as to define an unoccupied vertical interval within the fuel bundle starting at the top of the part length fuel rod and extending to the upper tie plate. These part length fuel rods have many advantages, which advantages are summarized in Dix et al. U.S. patent Ser. No. 07,176,975 entitled TWO-PHASE PRESSURE DROP REDUCTION BWR ASSEMBLY DESIGN now issued as U.S. Pat. No. 5,112,570 on May 12, 1992.